Calendar Calibration

Calibrated C-14 Dating Results

Calibrations of radiocarbon age determinations are applied to convert BP results to calendar years. The shortterm difference between the two is caused by fluctuations in the heliomagnetic modulation of the galactic cosmic radiation and, recently, large scale burning of fossil fuels and nuclear devices testing. Geomagnetic variations are the probable cause of longer term differences.

The parameters used for the corrections have been obtained through precise analyses of hundreds of samples taken from known-age tree rings of oak, sequoia, and fir up to about 10,000 BP. The parameters for older samples, up to about 20,000 BP, as well as for all marine samples, have been inferred from other evidence. Good calibrations are presently provided for samples to about 10,000 BP . From 10,000 to 20,000 BP approximate calibrations are possible, but subject to change in the future. Our calibrations are calibrated using the newest calibration data as published in Radiocarbon, Vol. 40, No. 3, 1998 using the cubic spline fit mathematics as published by Talma and Vogel, Radiocarbon, Vol. 35, No. 2, pg 317-322, 1993: A Simplified Approach to Calibrating C14 Dates. It is always important to quote the data source and mathematics with your interpretations.

The program chosen for these dendrocalibrations uses splines through the tree-ring data as calibration curves, which eliminates a large part of the statistical scatter of the actual data points and gives a better "real" approximation of the sample's calendar equivalence using the measured correlation curve. The spline calibration allows adjustment of the average curve by a quantified closeness-of-fit parameter to the measured data points.

Each result is provided in text form and graphic form on a single hard-copy (example below). On the calibration curve print-outs, the solid bars represent one sigma statistics (68% probability) and the hollow bars represent two sigma statistics (95% probability). Marine carbonate samples that have been corrected for dC13 and have also been corrected for both global and local geographic reservoir effects (as published in Radiocarbon, Volume 35, Number 1, 1993) prior to the calibration. Marine carbonates that have not been corrected for dC13, have been adjusted by an assumed value of 0 % in addition to the reservoir corrections. Reservoir corrections for fresh water carbonates are usually unknown and are generally not accounted for in those calibrations. In the absence of measured dC13 ratios, a typical value of -5 % was assumed for freshwater carbonates. Variables used in each calibration are listed below the title of each calibration page.

(Caveat: the calibrations assume that the material dated was living for exactly ten or twenty years (e.g. a collection of 10 or 20 individual tree rings taken from the outer portion of a tree that was cut down to produce the sample in the feature dated). For other materials, the maximum and minimum calibrated age ranges given by the computer program are uncertain. The possibility of an "old wood effect" must also be considered, as well as the potential inclusion of some younger material in the total sample. Since the vast majority of samples dated probably will not fulfill the ten/twenty-year-criterium and, in addition, an old wood effect or young carbon inclusion might not be excludable, dendro-calibration results should be used only for illustrative purposes. In the case of carbonates,
View a Sample Printout    reservoir correction is theoretical and the local variations are real, highly variable and dependant on provenience. The age ranges and, especially, the intercept ages generated by the program must be considered as approximations.)